SiC (silicon carbide) has a wide bandgap and is considered promising as a high temperature, high frequency, and high output semiconductor material. SiC includes many crystal polytypes. Among them, the crystal polytype 4H SiC is noteworthy as a semiconductor material with the wide bandgap and superior in the above characteristics.
For commercialization of SiC semiconductor devices, measurement of the carrier mobility by the Hall characteristics and other evaluation of characteristics are necessary. For correct measurement, it is necessary to form a good Ohmic contact. However, the 4H polytype has a large bandgap as explained above, so obtaining Ohmic characteristics is difficult in comparison with other crystal polytypes. Further, in the case of P-type SiC, until now, no art for forming stable Ohmic contacts has been established.
In the past, Ohmic contacts using Ti/Al or Al/Ti/Al stacked structures have been developed.
For example, B. J. Johnson, M. A. Capano, Solid-State Electronics 47 (2003), 1437-1441 discloses the formation of contacts with small contact resistance of a Al/Ti/Al triple layer structure on P-type 4H—SiC. However, to realize Ohmic characteristics, heavy doping is necessary in the vicinity of the contact regions of the SiC. Along with it, various problems occur. First, in the heavily doped state, the characteristics inherent to SiC cannot be confirmed and the fundamental information for device design cannot be obtained. Further, when for example employing ion implantation for the heavy doping, heat treatment of at least 1800° C. is necessary in order to repair damages occurring in the SiC crystal. However, this is a high temperature in comparison to the 1600 to 1700° C., the temperature of formation of the SiC thin film by CVD and the like being liable to degrade the thin film quality and have a detrimental effect on the characteristics of thin film devices. Further, treatment steps including an ion implantation step and a repair step become excessively necessary. This complicates the device production process and makes it costly.
As an electrode formed on SiC with a structure similar to this, Japanese Patent Publication (A) No. 2005-277240 discloses an Ohmic contact comprised of Ni/Ti/Al layers on a SiC wafer. However, the carrier concentration is raised to secure the Ohmic characteristics, so this cannot be applied to evaluation of characteristic at low carrier concentrations such as in single crystal wafers and epitaxial layers.
Further, Japanese Patent Publication (A) No. 2000-101064 discloses a Ti, Al, or other metal electrode on a SiC wafer through Nb-doped SrTiO3 or another conductive oxide. However, crystal polytypes are not shown. The possibility of application to the 4H-type is unclear.
Further, Japanese Patent No. 2911122 discloses to form a metal film exhibiting a stronger reaction to oxygen than P-type SiC by stacking an Al/Si Ohmic contact on top, then cause diffusion of the Al or Si by heat treatment. However, this raises the carrier concentration and lowers the contact resistance, so it is suitable for device contacts, but cannot be used for Hall measurement for evaluating the semiconductor characteristics inherent to SiC.